Gene expression of Epstein-Barr virus (EBV) was studied after microinjection of viral DNA into different types of cells. Raji TK- cells, known to express viral gene functions after superinfection with the EBV-P3HR-1 virus strain, were attached to plastic dishes by using anti-lymphocyte IgG, phytohemagglutinin, or concanavalin A as a ligand. It was difficult to inject DNA into the small and fragile Raji cells. After formation of polykaryons by cell fusion, microinjection became more efficient. At 24 hr after injection of P3HR-1 virus DNA, 90-100% of the injected cells expressed the early antigen complex as observed by immunofluorescence staining; 70-80% of the cells simultaneously incorported [3H]thymidine, indicating that thymidine kinase is expressed after injection of viral DNA. Additionally, synthesis of the virus capsid antigen was demonstrated in 20-30% of the recipient Raji cells. Human diploid fibroblasts, African green monkey kidney cells, and rat fibroblasts, which do not represent natural target cells for EBV, could also be induced to synthesis of early antigen complex by injection of P3HR-1 virus DNA.